The present invention relates in general to a spraying apparatus and method for mixing a flowable dry material with a liquid and for discharging the mixture against a surface to be coated. The invention finds particular utility in mixing cellulose fibers, which may contain various additives, with a liquid, which may include an adhesive, and spraying the resulting mixture against a desired surface, such as a building wall or ceiling, or the like, to produce an insulating coating. As a matter of convenience, such an application of the invention will be considered herein without necessarily limiting it specifically thereto.
The prior art is replete with examples of apparatuses for mixing dry materials and liquids and spraying the resulting mixtures against surfaces to be coated. The dry materials may range from portland cement, or mixtures thereof, to fibrous materials comprising mineral or cellulosic fibers. Similarly, the liquids may range from water to liquid adhesives.
Apparatuses which premix the dry and liquid materials and then pump the mixture to a spray nozzle simply will not work with fibrous materials, and particularly with cellulosic materials, because the pumping action breaks the mixture delivered to the discharge nozzle down into slugs of fibrous material and liquid, an obviously undesirable condition.
Another prior approach is to discharge the fibrous material in a dry state from a nozzle and then spray the liquid into the emerging stream of dry material externally of the nozzle, i.e., downstream from the downstream end thereof. This procedure works relatively well for a dry material comprising mineral fibers, but does not operate entirely satisfactorily with cellulosic fibers since the wetting of such fibers is not sufficiently uniform to preclude impingement of dry balls of the cellulose-base material on the surface being coated, a condition which again is obviously undesirable since it leads to nonuniformity of such things as thickness, density, consistency, appearance, and the like. Also, there may be considerable overspraying and resultant waste.
Other prior art attempts to wet the dry material while flowing through the discharge nozzle, or a passage leading thereto, utilize such expedients as venturis, liquid ejectors centrally located in the confined stream of dry material, and the like. Such expedients have not proven satisfactory, particularly with dry materials containing cellulose fibers, for various reasons, the principal one being nonuniform wetting. Central liquid ejectors and venturis suffer the additional disadvantage of providing obstructions to the flow of the dry material. Nevertheless, internal wetting, i.e., injection of the liquid into a confined stream of the dry material, has the potential for more uniform, and, equally important, more extensive, wetting of the dry material than external wetting, i.e., wetting of the dry material after it emanates from the discharge nozzle.